In a medium saucepan, combine the 1/3 cup sugar and cornstarch. Stir in milk, chocolate and the 1 tablespoon butter.,Cook and stir until thickened and bubbly.,Reduce heat; cook and stir 2 minutes more.,Gradually stir about 1 cup of the hot mixture into eggs. Return mixture to saucepan.,______,Cook and stir 2 minutes more.,Stir in vanilla.,Turn hot pie filling into baked pie shell.